NHS Choices - Preventing syphilis
The only guaranteed way to prevent a syphilis infection is to avoid sexual contact or only to have sexual contact with a faithful partner who has been tested and is not infected.
Condoms can reduce your risk of catching syphilis, but cannot prevent it altogether. You can still catch syphilis if your mouth makes contact with a sore on an infected person's anus or vagina, for example.
It is important not only to use a condom during vaginal, oral and anal sex, but also consider using a dental dam (square of plastic) when your mouth makes contact with your partner's vagina or anus. This will reduce your risk of any sexually transmitted infection (STI), not just syphilis.
Avoid sharing sex toys. If you do share them, wash them or cover them with a condom before each use.
Sexual penetration or ejaculation does not need to take place for syphilis to spread.
If you are an injecting drug user, do not use other people's needles. Many pharmacies and local authorities offer needle-exchange programmes, where used needles can be exchanged for clean ones. Your GP or drug counsellor should be able to provide more information.